1. Field of the Invention
This invention relates generally to an improvement in diagnostic medical ultrasound imaging, and more specifically to reducing elevation side lobe image artifacts.
2. Description of the Prior Art
There is growing interest in 2-D ultrasound images for medical applications. Medical ultrasound systems are described in U.S. Pat. No. 5,570,691, U.S. Pat. No. 5,667,373, U.S. Pat. No. 5,685,308, U.S. Pat. No. 5,827,188 and U.S. Pat. No. 5,882,307 to Wright, et al.; and U.S. Pat. No. 5,856,955 and U.S. Pat. No. 5,675,554 to Cole, et al. The theory of medical ultrasound imaging is extensively discussed in Physics and Instrumentation of Diagnostic Medical Ultrasound, by Peter Fish, pp. 1-189 (1990).
A medical ultrasound system includes a transmit beamformer to send electrical signals to an ultrasound transducer, which converts these electrical signals into acoustic pressure waves. The ultrasound transducer also converts received acoustic pressure wave signals into electrical signals to be processed by a receive beamformer and a signal processor. The ultrasound transducer is typically a 1-D ultrasound phased array transducer that provides the electrical signals that are ultimately processed to generate a 2-D image on a display device, where, in a typical B-mode image, each pixel location within the image corresponds to a spatially localized region being imaged. The brightness or color that is assigned to each pixel in the image is a function of the amplitude of the signal received by the 1-D ultrasound phased array transducer from the corresponding region. The amplitude of the received signal is also dependent on the transmission and reception characteristics of the ultrasound transducer itself.
A 1-D ultrasound phased array transducer typically comprises 64 to 128 elements in a linear array, on a 0.1 to 0.3 millimeter (mm) element-to-element pitch. The elements are typically between 2 and 14 mm in the elevation dimension (i.e., perpendicular to the line direction of the linear array, otherwise referred to as the azimuthal direction). Typically, all phased array transducers are subdivided in the azimuthal direction by dicing during array assembly, resulting in independent array elements that enable electronic steering, focusing, and dynamic apodization. However, these independent array elements are not perfectly unidirectional transducers, because such transducers frequently have transmission and reception image sensitivities at angles to their main direction sensitivity. An image sensitivity at an angle to the main imaging direction of the transducer element is called a side lobe.
A phased array transducer element with significant imaging side lobes is sensitive to echoes from structures outside the main beam, because the side lobe echoes would be interpreted and displayed by the ultrasound system as though received from a structure in the main beam direction. This spurious image is called a side lobe image artifact. Such image artifacts can be hard to distinguish from true images. In fact, a strong reflector located in the same direction as a side lobe could generate an image with an intensity equal to the intensity of an image generated by a weak reflector located in the direction of the main beam.
Proper amplitude apodization of the aperture reduces or nearly eliminates side lobes with their associated image artifacts. Apodization of the elevation aperture is always difficult to achieve, since the aperture is not normally subdivided in that direction. The only existing method in the prior art involves subdividing the elevation aperture, which increases the cost and complexity of the associated transducer and system. Apodization of the azimuthal aperture is either not attempted in low cost ultrasound systems, or is accomplished in higher cost ultrasound systems by expensive electronic circuitry.
What is needed is a relatively low cost apparatus and method that will achieve the correct apodization to reduce side lobes and side lobe image artifacts by offsetting the negative apodization of an elevation geometric lens. What is also needed is an apparatus and method to achieve a better apodization aperture, compatible with an ultrasound transducer probe assembly having a conventional, uniform thickness PZT design, or any other design.